This invention is directed to a thermal printer and, in particular, to an improved construction of a thermal printing head for use in thermal printers.
Thermal printers such as that described in U.S. Pat. No. 4,088,214 which is incorporated by reference herein as though fully set forth herein, are generally smaller in size and simplier in construction than conventional mechanical printer mechanisms which have, for example, print wheels including types which strike a recording medium. Thus, thermal printers are readily adaptable for use and can be incorporated as the output printer in small-sized pocket electronic calculators and the like. Thermal printers include a thermal printing head which selectively applies heat to a thermosensitive recording medium to print characters thereon. A particular construction and the operation of such a thermal printing head is described in U.S. Pat. No. 4,048,470 which is incorporated by reference herein as though fully set forth herein. See also U.S. Pat. No. 3,512,158. Conventional thermal printing heads do not provide a high printing quality since it has proven difficult to bring the thermosensitive recording paper into close contact with the heating elements on the printing head which form dots on the thermosensitive paper to form the characters thereon.
In order to be practically feasible, pocket electronic calculators require a small-sized thermal printer and cell (battery) utilized as a power supply. The cell can be, for example, a SUM-3 type or the smaller SUM-4 type manganese cell. The manganese cell is less costly than alkali-manganese and nickel-cadmium cells, but has an internal resistance of about 0.5.OMEGA./cell which is relatively large as compared with the alkali-manganese cell that has an internal resistance of about 0.4.OMEGA./cell and the nickel-cadmium cell that has an internal resistance of about 0.02.OMEGA./cell.
Accordingly, the present invention provides a printing head for a thermal printer having improved printing quality which is readily adaptable for use in pocket electronic calculators and the like since it requires less power to operate. The improved printing head of the present invention is small-sized, yet provides a good printing quality which allows a thermal printer incorporating such an improved printing head to be small in size.